1. Field of the Invention
This invention relates to an apparatus and a method for transforming picture information to be suitably used for processing picture information (bit stream) compressed by means of an orthogonal transform technique such as discrete cosine transformation (DCT) and a motion compensation technique conforming to the so-called MPEG Standards when receiving the information by way of a network media such as satellite broadcasting, CATV or Internet or a storage media such as an optical disk or a magnetic disk.
2. Related Background Art
In recent years, apparatus conforming to the so-called MPEG Standards and adapted to encode picture information into digital data typically by means of an orthogonal transform technique and a motion compensation technique, utilizing the redundancy specific to picture information, are becoming popular for the purpose of both dissemination of information by broadcasting stations and reception of information by family members so that information may be transmitted and stored in a highly efficient way.
Particularly, the so-called MPEG-2 (ISO/IEC 13818-2) Standards are established for general purpose video coding systems and cover images obtained by interlaced scanning and progressive scanning as well as images with an ordinary resolution and high definition images so that they are expected to broaden the applicable scope both in the professional field and consumer applications.
The so-called MPEG-2 compression system can realize a high compression ratio and an excellent image quality by assigning 4 to 8 Mbps to an image obtained by interlaced scanning with an ordinary resolution and typically having 720xc3x97480 pixels and 18 to 22 Mbps to an image obtained by interlaced scanning with the high resolution and typically having 1,920xc3x971,088 pixels.
However, the quantity of information of a high definition image is enormous and the currently available coding systems including those conforming to the MPEG Standards are still accompanied by the problem of meeting the requirement of a bit rate of 18 to 20 Mbps or more for realizing a satisfactory image quality for an image obtained by interlaced scanning of 30 Hz and having 1,920xc3x971,080 pixels.
Therefore, it is necessary to further reduce the bit rate, taking into consideration the band width of the transmission path in the case of transmitting information by way of a network media such as satellite broadcasting or CATV or the storage capacity in the case of storing information by using a storage media schematic as optical disk or magnetic disk, while minimizing the possible degradation of image quality.
The necessity of reducing the bit rate may arise when processing the compressed picture information (bit stream) of a high definition image but also that of an image with an ordinary resolution (e.g., an image obtained by interlaced scanning of 30 Hz and having 720xc3x97480 pixels) for a network media or a storage media as described above.
Techniques of hierarchical coding (scalability) and picture information transform (transcoding) have been proposed as means for solving the above identified problem.
With regard to hierarchical coding (scalability), the SNR (signal to noise ratio) scalability is standardized in MPEG-2 so that it is possible to hierarchically encode high SNR compressed picture information (bit stream) and low SNR compressed picture information (bit stream) according to it.
However, while the restrictions in terms of band width or storage capacity have to be known at the time of coding for hierarchical coding, they are practically unknowable in almost any actual situations.
From this point of view, picture information transform (transcoding) may find more practical applications in actual systems.
An picture information transform apparatus (transcoder) basically comprises a decoder for entirely or partly decoding the input compressed picture information (bit stream) and an encoder for re-encoding the output of the decoder that are connected in parallel and information (pixel data) may be delivered from the decoder to the encoder either in a space region or in a frequency region.
While the arrangement of delivering information (pixel data) in a space region requires a large quantity of data to be processed, the degradation of the quality of the decoded image obtained from the output compressed information (bit stream) can be minimized. This arrangement find applications mainly in broadcasting equipment.
On the other hand, the arrangement of delivering information (pixel data) in a frequency region requires less data to be processed so that it finds applications mainly in home use equipment, although the image quality may be degraded to a greater extent if compared with the former arrangement.
Meanwhile, known picture information transform apparatus are accompanied by the problem of degradation of image quality such as block distortion as a result of limiting the bandwidth in order to reduce the bit rate of compressed picture information (bit stream).
Additionally, known picture information transform apparatus are not satisfactory in terms of the extent of reduction of the bit rate of compressed picture information (bit stream).
In view of the above identified problems, it is therefore the object of the present invention to provide a method and an apparatus for transforming picture information that can satisfactorily suppress the degradation of coding quantity due to a limited bandwidth and reduce the bit rate of compressed picture information.
According to the invention, the above object is achieved by providing a picture information transform apparatus for receiving as input the compressed picture information of a plurality of picture type obtained by coding the picture information by means of orthogonal transform and motion compensation, said apparatus comprising:
a decoding means for decoding the input compressed picture information to the picture information;
a coding means for coding the picture information obtained by the decoding of said decoding means; and
a control means for controlling the target coding quantity of each of the coded pictures of respective picture type by adaptively using a plurality of parameters to be used as coefficients of the contents of each of the coded pictures of respective picture type on the basis of the contents of each of the coded pictures of respective picture type at the time of coding by said coding means.
In another aspect of the invention, there is provided a picture information transform method for receiving as input the compressed picture information of a plurality of picture type obtained by coding the picture information by means of orthogonal transform and motion compensation, said method comprising:
a decoding step for decoding the input compressed picture information to the picture information;
a coding step for coding the picture information obtained by the decoding of said decoding step; and
a control step for controlling the target coding quantity of each of the coded pictures of respective picture type by adaptively using a plurality of parameters to be used as coefficients of the contents of each of the coded pictures of respective picture type on the basis of the contents of each of the coded pictures of respective picture type at the time of coding by said coding step.
Preferably, a picture information transform apparatus according to the invention for receiving as input the compressed picture information (bit stream) obtained by coding the picture information of a plurality of picture type by means of techniques of orthogonal transform and motion compensation typically conforming to co-called MPEG comprises a first code buffer, a compressed information analyser, an information buffer, a variable length decoder and inverse quantizer, a first adder, a band limiter, a quantizer, a coding quantity controller, a second buffer, a variable length encoder and a motion compensation error correcting section. The coding quantity controller includes a complexity computing section and a target coding quantity computing section and the motion compensation error correcting section includes an inverse quantizer, a second adder, an inverse discrete cosine transform section, a video memory, a motion compensation predicting section and a discrete cosine transform section. Thus, according to the invention, an optimal coding quantity is assigned to each frame of picture in the input compressed picture information (bit stream) according to the complexity of the frame in order to minimize the coding quantity (bit rate) of the output compressed picture information (bit stream) and also the distortions that can arise as a result of the re-encoding.
Of the above arrangement, the compressed information analyser extracts coded information necessary for the subsequent processing operations by syntactically analysing the compressed picture information (bit stream) and the variable length decoder performs a variable length decoding operation on the variable length coded discrete cosine coefficients according to the information on the scanning mode (zigzag scan or alternate scan) extracted by the compressed information analyser, while the inverse quantizer inversely quantizes the discrete cosine transform coefficients according to the information on quantization (quantization scale and quantization matrix) extracted by the compressed information analyser and the band limiter reduces the resolution of image by using 0s for the high frequency coefficients of the discrete cosine transform coefficients or multiplying them with a weighting coefficient. The quantizer requantizes the discrete cosine transform coefficients by using the width of quantization corresponding to the quantity of picture information (target bit rate) of the output compressed picture information (bit stream) and the variable length encoder performs a variable length coding operation on the discrete cosine transform coefficients, wile the coding quantity controller controls the given target coding quantity and the width of quantization of the quantizer as extracted from the information buffer such that no overflow or underflow of compressed picture information may occur in the second code buffer after the variable length decoding.
Particularly, the complexity computing section of the coding quantity controller computationally determines the complexity of each frame within the input compressed picture information (bit stream) on the basis of the number of bits assigned to the frame and stored in the information buffer and the quantization scale and the target coding quantity computing section of the coding quantity controller assigns an optimal coding quantity to each frame within the output compressed picture information (bit stream) on the basis of the complexity of the frame as determined by the complexity computing section.
If the arrangement includes a motion compensation error correcting section, the motion compensation error of each macro-block is stored in the video memory and then the stored error is taken out according to the information on the motion vector and the prediction mode as extracted from the input compressed picture information (bit stream).
As pointed out above, the present invention provides a picture information transform apparatus adapted to receive compressed picture information (bit stream) conforming to so-called MPEG-2 as input and deliver data from the decoder to the encoder thereof within a DCT region, with which the degree of degradation of coding quantity due output re-encoding of the compressed picture information (bit stream) to be output that conforms to so-called MPEG-2 and shows a coding quantity (bit rate) lower than that of the input information is reduced by assigning an optimal number of bits to each frame within the input compressed picture information (bit stream), taking the complexity of the frame into consideration.